Download The Segments and the Inferior Boundaries of the Odontoid Process

Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
The Segments and the
Inferior Boundaries of the
Odontoid Process of C2
Based on the Magnetic
Resonance Imaging Study
Keramettin AYDIN
Cengiz ÇOKLUK
Ondokuzmayıs University, Neurosurgery
Department, Samsun, Turkey
C2 Odontoid Ç›k›nt›n›n Segmentleri
ve ‹nferior S›n›rlar›n›n Manyetik
Rezonans Görüntüleme Temel
Al›narak Belirlenmesi
ABSTRACT
AIM: The aims of this clinical study were to describe the segments and the inferior
boundaries of the odontoid process as regards the embryological development of C2
based on the magnetic resonance imaging finding.
MATERIAL and METHODS: Cranial and cervical magnetic resonance images including
occiput, C1, and C2, and those obtained for different reasons such as evaluation of cranial
and upper cervical pathology were re-evaluated for this study. Synchondroses around the
odontoid process were accepted as the boundaries between the neural arch and the body
of C2.
Received: 02.11.2007
Accepted: 03.01.2008
RESULTS: Thirty cases were included for this study. Fifteen of these were adult cases and
the remaining 15 were in pediatric cases. Apicodental, dentocentral, neurocentral, and
dentoneural synchondrotic articulations were clear, especially under the ages of 3 years.
The dentocentral synchondrosis was found well below the line drawn through the level of
superior articulating facets.
CONCLUSION: This study demonstrated that the inferior boundary of the odontoid
process is not located at the level of superior articulating facets. The real border between
the odontoid process and the body of C2 is located well below the level of the superior
articulating facets because of the location of the dentocentral synchondrotic articulation.
This level should be considered in the classification of C2 fractures.
KEY WORDS: Odontoid process, Dentocentral synchonrosis, The boundaries of the
odontoid process
ÖZ
AMAÇ: Odontoid segmentleri ve inferior sınırlarının embriyolojik gelişme aşamaları
temel alınarak MR (Manyetik Rezonans) ile belirlenmesidir.
YÖNTEM ve GEREÇ: Oksiput, C1 ve C2’yi içine alan, kraniyal ve üst servikal değişik
patolojileri incelemek amacıyla çekilen detaylı ince kesitleride içeren manyetik rezonans
tetkikleri tekrar incelendi. Odontoid ile C2 korpusu ve nöral arkus arasındaki
sinkondrotik eklemler odontoid’in inferior sınırları olarak tesbit edildi.
BULGULAR: Bu çalışma 30 olguyu kapsamaktadır. Bu olgulardan 15’i erişkin yaş
grubunda iken geri kalan 15 olgu çocukluk yaş grubundaydı. Üç yaşın altındaki olgularda
apikodental, dentosantral, nörosantral ve dentonöral sinkondrotik eklemler belirgindi.
Dentosantral sinkondrosis süperior artiküler fasetler hizasından çekilen hattın altında
kalıyordu.
SONUÇ: Bu çalışma bize odontoid’in inferior sınırının superior artiküler fasetler
hizasında olmadığını göstermektedir. Odontoid ve C2 cismi arasındaki sınır süperior
artiküler faset seviyesinin daha aşağısında kalmaktadır, çünkü dentosantral sinkondrosis
faset seviyesinin altında yerleşiktir. C2 kırıklarının sınıflandırılmasında bu göz önünde
bulundurulmalıdır.
ANAHTAR SÖZCÜKLER: : Odontoid çıkıntı, Dentosentral sinkondrosis, Odontoid
çıkıntının sınırları
Correspondence address:
Keramettin AYDIN
Ondokuzmayıs Üniversitesi Tıp Fakültesi
Nöroşirürji AD., Kurupelit, Samsun
E-mail : [email protected]
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Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
INTRODUCTION
The second cervical vertebra is unique in terms of
anatomical shape, function, and biomechanical
properties (2,3). The functions of the head such as
flexion, extension, lateral bending, and lateral
rotation is related to the anatomical function of C2
and its odontoid process. C2 is also the largest
among the members of upper cervical spine (2). It
consists of a body, paired pedicles, lateral masses
(superior articulating facets, pars interarticularis,
and inferior articulating facets), laminae, and bifid
spinous process (2,3,5,12). Other members of the
upper cervical spine include the foramen magnum,
paired occipital condyles and C1 (5,9). This segment
serves as a transitional zone between the rigid
calvarium and flexible lower cervical spine
(2,3,5,12).
The odontoid process of C2 projects upward from
the superior roof of the body, and differs it from the
others (2,5,12). In embryological developmental
stages, C2 forms from four bones separated by
synchondrotical articulations and consisting of four
ossification centers (two of them are located in the
neural arches bilaterally, one of them is located in the
body, and one is located in the odontoid process).
The borders of the odontoid process are well
demarcated by these cartilaginous articulations
during prenatal and postnatal development of C2
until the end of the ossification process (2). These
cartilaginous articulations are named dentocentral
(separating odontoid from the body) and
neurocentral synchondrosis (separating odontoid
and body from neural arches) (2,7). Synchondroses
among the neural arches, body, and odontoid
process fuse at 3 to 6 years (2). After the age of 6
years, the odontoid process fuses with the body and
the neural arches (2). In adult age, the remnant of the
dentocentral synchondrosis can be imaged by
magnetic resonance as a hypointense ring between
the inferior end of the odontoid and the superior
roof of the body of C2. This structure is located in the
cancellous bone, and should be accepted as the
inferior border of the odontoid process in adults. The
anatomical level of dentocentral synchondrosis is
well below the superior articulating facets and the
indentation of the transverse ligament to the
posterior aspect of the odontoid process.
C2 fractures make up a large percentage of all
cervical injuries. The fractures of the odontoid
process are relatively common among C2 fractures.
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Aydın: The Segments and the Inferior Boundaries of the Odontoid
Anderson and D’Alonzo (1) classified odontoid
fractures into three types. Type I occurs through the
upper part of the odontoid process, probably as a
result of an avulsion at the insertion of the alar
ligament. Type II fractures occur at the junction of
the odontoid process with the vertebral body. Type
III extends down into the body of the axis. In this
classification, the level of superior articulating facets
of the odontoid process is accepted as the border
between the odontoid process and the body of C2.
However, during embryological development there
is a dentocentral synchondrotic articulation between
the odontoid process and the body of C2. In adult
ages, the remnant of dentocentral synchondrosis
should be accepted as the inferior boundary of the
odontoid process of C2.
The aims of this clinical study were to describe
the segments and the inferior boundary of the
odontoid process the embryological development of
C2 based on the MRI study. Using MR images, we
will describe the remnant of dentocentral
synchondrosis. This remnant is located between the
inferior border of the odontoid process and the body
of C2.
MATERIALS and METHODS
The study population consisted of 15 adults (8
males aged between 18 and 71 years, and 7 females
aged between 22 and 68 years) and 15 pediatric
cases (9 males aged between 1 and 17 years, and 6
females aged between 2 and 15 years). The upper
cervical spine of all patients was examined by using
magnetic resonance imaging (MRI).
Neuroradiological images were obtained for the
evaluation of other suspicious pathologies. Were
selected images from the upper cervical and thin
slice MRI.
Magnetic resonance images were obtained using
a 1.0-tesla unit (General Electric) with a neck and/or
head surface coil. The MR imaging protocol for the
upper cervical region included 3-mm sagittal T1weighted (TR 600 msec, TE 20 msec) and T2weighted turbo spin-echo (TR 2500 msec, TE 80
msec) sequences.
Lateral and anterior odontoid radiographs were
obtained in all patients. All patients also underwent
Oc-C2 computerized scanning with sagittal
reconstructions. Coronal MR Images were obtained
in all patients. Midsagittal and midcoronal MR
images were scanned and converted into digital
Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
Aydın: The Segments and the Inferior Boundaries of the Odontoid
images by using a 3.2 megapixel digital camera
(Sony, Japan). All digital images were copied into the
hard-disc of a computer (Vestel Asteo computer,
Turkey).
The first stage of the study was the evaluation of
the patients under the age of 3 years. Synchondrotic
articulations were investigated with MRI. These
articulations were as follows; apicodental
synchondrosis,
dentocentral
synchondrosis,
neurodental synchondrosis, and neurocentral
synchondrosis. The neurodental, dentocentral and
neurocentral syncondrosis were selected as inferior
boundaries of the odontoid process. The
dentocentral synchondrosis is located between the
border of odontoid process and the body of C2. The
neurocentral synchondrosis is located between the
neural arch and the body of C2 and the neurodental
synchondrosis is located between the neural arch
and the odontoid process. The appearance of the
remnant of the dentocentral synchondrosis is shown
in Figure 1. Apicodental, dentocentral and
neurocentral synchondroses are shown in Figure 2.
The parts of C2 are shown in Figure 3.
RESULTS
Dentocentral synchondrosis in the pediatric age
and its remnant in adult ages were demonstrated in
all cases by using sagittal images of MRI.
Figure 2: Coronal T1-weighted magnetic resonance imaging of a
pediatric case shows the segments of the odontoid process (T: Tip
of the odontoid, N: The neck of the odontoid, Ba: The base of the
odontoid, Bo: The body of C2, Ap-Den: Apicodental
synchondrosis Neu-Den: Neurodental synchondrosis, dotted
areas mark the borders of the segments of the odontoid process).
Dentocentral and apicodental synchondroses have
different locations than the other synchondroses.
The apicodental synchondrosis is located in the
odontoid process. The dentocentral synchondrosis is
located between the odontoid process and the body
of C2. In other words, the dentocentral
synchondrosis is located at the superior border of the
body of C2 and inferior border of the odontoid
process.
In the pediatric cases under the age of 3 years, the
appearance of apicodental synchondrosis was very
clear. At the same time, the appearance of
dentocentral synchondrosis was also very clear in
cases under the age of 3 years. Synchondrotic
articulations were seen as hypointense bands in T1weighted MRI in all cases (Figure 2). These
articulations were seen as hyperintense bands in T2weighted MRI.
Figure 1: A. Coronal T2-weighted magnetic resonance imaging
in a 63-year-old adult shows the remnant of dentocentral
synchondrosis. This remnant should be accepted as the inferior
boundary of the odontoid process (arrow shows the remnant of
the dentocentral synchondrosis).
B. Sagittal T2-weighted magnetic resonance imaging of an adult
case shows the remnant of the dentocentral synchondrosis of C2
(arrow shows the remnant of the dentocentral synchondrosis).
The synchondrotic articulations were not seen in
all adult cases. The remnant of dentocentral
synchondrosis can only be seen in sagittal and
coronal MR images (Figure 1). The image
characteristics can be described as follows; in T1weighted MRI, these articulations can be imagined
as hypointense circular disc (Figure 2). In T2weighted MRI they were seen as hyperintense discs.
In the pediatric ages, the dentocentral synchondrosis
was accepted as the border of odontoid process with
the body of C2. In the same way the remnant of the
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Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
A
B
Figure 3: A. This figure taken from a cadaver C2 shows the parts
of C2 in antero-posterior view (O: Odontoid process, N: Neural
arc, B: Body of C2).
B. This figure taken from a cadaver C2 shows the parts of C2 in
posterio-anterior view (O: Odontoid process, N: Neural arc, B:
Body of C2).
dentocentral synchondrosis should be accepted as
the border of the odontoid process and the body of
C2 (Figure 1). In all cases the location of the remnant
of dentocentral synchodrosis was well below the line
drawn through the superior articulating facets.
We divided the odontoid process of C2 into three
segments according to the synchondrotic
articulations. The first segment is the tip of the
odontoid process. This segment is located in the
upper portion of the odontoid process. The border of
this segment is the apicodental synchondrosis. It is
impossible to see the apicodental synchondrosis in
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Aydın: The Segments and the Inferior Boundaries of the Odontoid
the adult ages. The border of the tip of the odontoid
is therefore an imaginary line drawn through the
upper portion of the odontoid at these ages. The
second portion is the neck of the odontoid process.
This segment begins from the end of the tip of the
odontoid process to the line drawn through the
superior articulating facets. According to our study,
the odontoid process ends at the remnant of
dentocentral synchondrosis. This line should
therefore not be accepted as the junction of the
odontoid with the body of C2. This area should be
accepted as the neck of the odontoid segment. The
last segment in our study is the base of the odontoid
process. This segment starts at the line drawn
through the level of superior articulating facets to the
remnant of dentocentral synchondrosis. The inferior
portion from the dentocentral synchondrosis should
be accepted as the body of C2 (Figure 4).
The lateral borders of the odontoid process were
very clear in the pediatric ages because of the
neurodental synchonrosis located between the
neural arc and the base of the odontoid process. We
divided this synchondrosis into two segments. The
first segment is the neurocentral synchondrosis. This
synchondrosis is located between the neural arc and
the body of C2. The second segment is the
neurodental synchondrosis. This segment is located
between the neural arc and the base of the odontoid
process. In adult ages, this is an imaginary line
drawn horizontally lateral to the odontoid process
bilaterally. This line passes from the pedicle of C2.
DISCUSSION
Fractures of the odontoid process make up a large
percentage of odontoid process pathologies.
Odontoid fractures were first described by Lambotte
in 1894. Following this description, odontoid
fractures were treated by non-surgical treatment
modalities for a long time. Mixter and Osgood first
surgically treated cases with odontoid fractures
(posterior fixation with wiring) in 1910 (4).
De Morgues and Fischer first classified odontoid
fractures as base and the neck fractures of the
odontoid process in 1972 (12). After this classification
Anderson and D’Alanzo proposed a new
classification system that is widely used in
neurosurgical practice in the world (1). This
classification system described three fracture types.
The original description of odontoid fractures
according to Anderson and D’Alanzo classification
Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
Aydın: The Segments and the Inferior Boundaries of the Odontoid
odontoid process. The area of Type II fracture is a
horizontal line drawn through the upper border of
the superior articulating facets of the axis. Anderson
and D’Alanzo and some other authors described this
line is the junction of the odontoid process with the
body of C2.The same classification described the
fractures that pass below this level as body of C2
fractures (Type III fractures). It is necessary to know
the inferior boundary of the odontoid process to
accept these descriptions. In this clinical study, we
described the inferior boundary of the odontoid
process as the remnant of dentocentral
synchondrosis by using MRI. According to our
study, the imaginary line drawn through the upper
border of the superior articulating facets of the axis is
not the junction of the odontoid process and the
body of C2. The remnant of the dentocentral
synchondrosis is well below this level. At the same
time, the fractures passing under this horizontal line
should not be accepted as the body of C2 fracture
because of the location of the dentocentral
synchondrosis remnant.
A
B
Figure 4: A. This figure taken from a cadaver C2 shows the
segments of the odontoid process in antero-posterior view (T: Tip
of the odontoid process, N: Neck of the odontoid process, Ba: Base
of the odontoid process, Bo: Body of C2, NA: Neural arc, dotted
areas mark the borders of the segments of the odontoid process)
B. This figure taken from a cadaver C2 shows the segments of the
odontoid process in posterior-anterior view (T: Tip of the
odontoid process, N: Neck of the odontoid process, Ba: Base of the
odontoid process, Bo: Body of C2, NA: Neural arc, dotted areas
mark the borders of the segments of the odontoid process)
can be addressed as follows. Type I fracture occurs
through the upper part of the odontoid process,
probably as a result of an avulsion at the insertion of
the alar ligament. Type II fractures occur at the
junction of the odontoid process with the vertebral
body. Type III extends down into the body of the
atlas. The mentioned descriptions reflect the original
description of Anderson and D’Alanzo. At the first
glance, there seems to be no problem for these
descriptions. In fact, there are some anatomical
problems with Type II and Type III fractures of the
Stillerman et al (12), in their classical monograph,
defined the line at the level of the superior
articulating facets as the synchondrosis and stated
that the majority of odontoid fractures involve this
line where the odontoid process fuses with the body
of C2. In fact, this line is just the border between the
neck and the base of the odontoid segment, and the
real dentocentral synchondrosis is located between
the base of the odontoid and the body of C2, and
more below the neck of the odontoid.
The external surface anatomy of C2 had been
studied and well-described previously in many
anatomical studies (2,3,6,9,10,11). The internal
anatomy of the vertebral bones, especially the
internal trabecular anatomy of C2 was studied in
only a few published articles regarding the aspect of
biomechanical properties and segmental anatomy (2,
8). It is necessary to know the internal anatomy of C2
for proper diagnosis, description and classification.
MRI is a useful neuroradiological diagnostic tool
in the investigation of the internal cancellous
anatomy of the vertebral bones in living subjects at
different ages. Our study revealed that MRI has a
high capability to demonstrate the remnant of the
dentocentral synchondrosis in both coronal and
sagittal images. CT with bone window can show the
cortical bone structure in the odontoid and body
segments. It is however difficult to distinguish the
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Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
remnant of neurocentral synchondrotic articulations;
the remnant of dentocentral synchondrosis was seen
as a hypointense disc in T1-weighted images located
midline between the space of odontoid and the body
of C2. In T2-weighted MR images, this area was seen
as a hypointense area.
This clinical study demonstrated that the
remnant of the dentocentral synchondrosis is located
well below the superior articulating facets and can
be imagined by sagittal and coronal magnetic
resonance images. This remnant can be seen in
pediatric patients, adults, and even in the elderly.
The localization and level of the remnant of the
dentocentral synchondrosis is not only important as
an anatomical detail but it is also extremely
important from the clinical perspective aspect
because of odontoid and C2 fractures. The odontoid
segment may be divided into three segments
according to the localization of the dentocentral
synchondrosis. The first segment is the tip of the
odontoid. This segment is separated from the second
segment of the odontoid (the neck) by the
apicodental synchondrosis. The neck of the odontoid
process lies between the apicodental synchondrosis
and the level of the superior articulating facets. The
third segment connects the neck to the body of C2
via the dentocentral synchondrosis, and can be
named the base of the odontoid segment. This
segment also fuses with the neural arches bilaterally.
In the stage of embryological development and in
pediatric ages the inferior boundary of the odontoid
process was formed by the synchondrotic
articulations between the parts of C2 based on the
magnetic resonance imaging findings.
The neural arc, the body and the odontoid
process are three main parts of C2. The odontoid
process makes articulations with all these structures.
The dentocentral synchondrosis is located between
the inferior end of the odontoid process and superior
border of the body of C2. In pediatric ages it is very
clear and easy to distinguish this articulation. In
adult ages, with the regression of dentocentral
synchondrosis the remnant of dentocentral
syncondrosis appears. This remnant is very
important in the description of the inferior boundary
of the odontoid process in adult ages. Another
synchondrosis is the neurodental syndchondrosis.
This synchondrosis is located between the
inferolateral boundary of odontoid process and the
neural arc. In pediatric ages this articulation is very
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Aydın: The Segments and the Inferior Boundaries of the Odontoid
clear, but in the adult ages it disappears. The
imaginary line drawn medial border of the superior
articulating facets can be accepted as the inferolateral
boundary of the odontoid process. This area is
generally accepted as the pedicle of C2.
According to our description of the inferior
boundary of the odontoid process of C2 based on the
synchondrotic articulations between the parts of C2,
it is necessary to re-describe the location of the
fracture of the odontoid process. As mentioned
previously we divided the odontoid process into
three segments. The first segment is the apical
segment or the tip of the odontoid process. This
segment starts at the tip of the odontoid process and
ends at the apicodental synchondrosis in pediatric
ages. In adult ages, this level is just an imaginary line
drawn horizontally at the superior part of the
odontoid process. Type I fractures of odontoid
process are seen in this location. The second segment
is the neck of the odontoid process. This segment
begins from the apicodental synchondrosis in
pediatric ages and imaginary line in adults as
mentioned above. This segment ends at the
imaginary line drawn through the superior end of
the superior articulating process. Some authors have
mistakenly described this segment as the junction of
the odontoid process with the body of C2. Actually
this is just inside part of the odontoid process. The
fractures seen in this location are accepted as Type II
fractures. After our description, these fractures can
be accepted as the neck fractures of the odontoid
process. The third and the last segment of the
odontoid process is the base of the odontoid process.
This segment is located between the imaginary line
drawn at the superior level of the superior
articulating process and the dentocentral
synchondrosis in pediatric ages. In adult ages the
inferior level can be described as the remnant of the
dentocentral synchondrosis. This remnant can be
seen in MRI as described previously in this text. The
fractures in this segment can be described as the base
fractures of the odontoid process. According to our
description of the inferior boundary of the odontoid
process, the fractures seen in this location are not the
C2 body fractures, as the body of C2 is located under
the level of the dentocentral synchondrosis remnant.
CONCLUSION
Based on the localization of the dentocentral
synchondrosis, the odontoid process of C2 can be
divided into three segments as mentioned above; the
Turkish Neurosurgery 2008, Vol: 18, No: 1, 23-29
tip, the neck, and the base. The base of the odontoid
process begins from the level of superior articulating
facets, and ends at the level of the remnant of the
dentocentral synchondrosis. The level of superior
articulating facets should be considered as a border
inside the odontoid process separating the neck from
the base of the odontoid segments. Fractures of the
odontoid process should be redefined according to
this evaluation.
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